The mechanism of catalysis and electron transfer in flavocytochrome b2 will be studied by x-ray diffraction methods. Several mutant forms of the enzyme with interesting properties are available for study and their structures will be compared with the native form. In addition studies of the native and mutant forms of the enzyme will be carried out with various inhibitors bound at the active site and in several oxidation states. Other studies will concentrate on preparing a crystalline complex of flavocytochrome b2 with cytochrome c, crystallizing and analyzing the isolated cytochrome b2 core domain and completing the high resolution refinement of cytochrome b562. Finally crystallographic studies of a flavoprotein oxidase, hydroxyacid oxidase will be initiated. Flavocytochrome b2 from yeast mitochondria is a tetrameric bifunctional enzyme containing one heme and one FMN prosthetic group per subunit of Mr 57,500. It catalyses the oxidation of lactate to pyruvate and the reduction of cytochrome c. Its structure has been determined to 2.4 A resolution. Each subunit contains 2 domains, a cytochrome domain resembling cytochrome b5 and a flavin-binding domain folded into a parallel beta 8 alpha 8 barrel motif. In 2 of the 4 subunits of the crystals the cytochrome domain is disordered and the corresponding flavin-binding domain contains pyruvate bound at the active site. Hydroxyacid oxidase from rat kidney, a tetramer of subunit Mr 39,000, contains FMN and is similar in many respects to flavocytochrome b2. However, the terminal electron acceptor is molecular oxygen rather than a cytochrome. Cytochrome b562, of Mr 12,500 is a 4-alpha helical bundle containing noncovalently bound heme. Its structure has been solved at 2.5 A resolution and the x-ray data extended to 1.4 A resolution.